As is known in the art, an alternator is an alternating current (ac) output generator. To convert the ac voltage to direct current (dc) for use in charging batteries or supplying dc loads, for example, a rectifier system is used. Sometimes, the alternator is referred to as an ac machine or more simply a machine and the combined machine/rectifier system is referred to as an alternator or an alternator system.
In many cases (including automotive alternators), a diode rectifier is used to rectify the ac voltage produced by the generator. The ac machine can be modeled as a three-phase voltage source and a set of inductors.
In a so-called wound-field machine, the output voltage or current can be controlled by varying the current in a field winding which in turn varies the ac voltage magnitudes. The advantage to this approach is the extreme simplicity and low cost of the system. One particular type of wound field machine is a so-called wound-field Lundell-type alternator. A Lundell machine is characterized by the way the rotor/field of the machine is constructed, the details of which are well-known to those of ordinary skill in the art. Significantly, the construction techniques used to manufacture Lundell-type alternators result in an ac machine which is relatively inexpensive but which has a relatively high inductance or reactance. Wound-field Lundell-type alternators are almost universally used in the automotive industry primarily because they are reliable and inexpensive. One problem with wound-field Lundell-type alternators, however, is that the relatively high machine inductance strongly affects the machine performance. In particular, due to the high inductance of the Lundell machine, it exhibits heavy load regulation when used with a diode rectifier. That is, there are significant voltage drops across the machine inductances when current is drawn from the machine, and these drops increase with increasing output current and machine operating speed. Consequently, to deliver substantial current into a low dc output voltage, the ac machine voltage magnitudes have to be much larger than the dc output voltage.
In a typical high-inductance automotive alternator operating at relatively high speed, the internal machine voltage magnitudes are in excess of 80 V to deliver substantial current into a 14 V dc output. This is in contrast with a low-reactance machine with a diode rectifier, in which the dc output voltage is only slightly smaller than the ac voltage magnitudes.
One approach to controlling alternator output voltage is to utilize a field current regulator as shown in FIG. 1A. In this approach, the field current if of a machine 10 is determined by a field current regulator 12 which applies a pulse-width modulated voltage across the field winding. The armature of the machine 10 is modeled as a Y-connected set of three-phase back emf voltages vsa, vsb, and vsc and leakage inductances Ls. A fundamental electrical frequency ω (fundamental electrical cycle) is proportional to the mechanical speed ωm and the number of machine poles in the machine 10. For example, the fundamental frequency of an alternator having four machine poles (two pairs of poles) rotating at a frequency of 3600 rpm (60 revolutions per second) would be two times the rotational frequency or 120 cycles per second which is the basic period of the ac voltage generated by the machine 10 prior to rectification.
The magnitude of the back emf voltages is proportional to both frequency and field current. A diode bridge 14 rectifies the ac output of the machine 12 to provide a constant output voltage Vo (perhaps representing a voltage across a battery and associated loads). This simple model captures many of the important characteristics of conventional alternators, while remaining analytically tractable, as described in V. Caliskan, D. J. Perreault, T. M. Jahns and J. G. Kassakian, “Analysis of three-phase rectifiers with constant-voltage loads,” IEEE Power Electronics Specialists Conference, Charleston, S.C., June-July 1999, pp. 715-720 and in D. J. Perreault and V. Caliskan, “Automotive Power Generation and Control,” LEES Technical Report TR-00-003, Laboratory for Electromagnetic and Electronic Systems, Massachusetts Institute of Technology, Cambridge, Mass., May 24, 2000.
Another approach to controlling output voltage or current is to utilize a controlled rectifier rather than a field current regulator. One simple and often-used approach for controlled rectification is to replace the diodes of a diode rectifier with thyristor devices. For example, as described in J. Schaefer, Rectifier Circuits, Theory and Design, New York: Wiley, 1965 and in J. G. Kassakian, M. F. Schlecht, and G. C. Verghese, Principles of Power Electronics, New York: Addison-Wesley, 1991, thyristor devices can be used in a semi-bridge converter. With this technique, phase control (i.e. the timing of thyristor turn on with respect to the ac voltage waveform) is used to regulate the output voltage or current. One problem with this approach, however, is that it can be relatively complex from a control point of view. This is especially true when the alternator must provide a constant-voltage output.
Alternatively, rather than using field control or phase control, another approach to controlling output voltage or current is to utilize switched-mode rectification (SMR). With the switched-mode rectification technique, fully-controllable switches are used in a pulse width modulation (PWM) fashion to produce a controlled dc output voltage from the ac input voltage. This approach, which typically utilizes a full-bridge converter circuit, often yields high performance at the expense of having many fully-controlled PWM switches and complex control circuits and techniques.
One relatively simple switched-mode rectifier that has been employed for alternators attached to wind turbines is described in an article entitled “Variable Speed Operation of Permanent Magnet Alternator Wind Turbines Using a Single Switch Power Converter,” by G. Venkataramanan, B. Milkovska, V. Gerez, and H. Nehrir, Journal of Solar Energy Engineering—Transactions of the ASME, Vol. 118, No. 4, November 1996, pp. 235-238. In this approach, the alternator includes a rectifier comprising a diode bridge followed by a “boost switch set” provided from a controlled switch such as a metal oxide semiconductor field effect transistor (MOSFET) and a diode. The switch is turned on and off at a relatively high frequency in a PWM fashion. This approach is utilized along with PWM switching generated by a current-control loop to simultaneously control the output current and turbine tip speed of a permanent magnet alternator. The approach is specifically applied to a low-reactance (i.e. low-inductance) permanent-magnet ac machine where the battery voltage is higher than the ac voltage waveform. It should be noted that the rectifier system is topologically the same as the Discontinuous Conduction Mode (DCM) rectifier described in an article entitled “An Active Power Factor Correction Technique for Three-Phase Diode Rectifiers,” by A. R. Prasad, P. D. Ziogas, and S. Manias, the IEEE Trans. Power Electronics, Vol. 6, No. 1, January 1991, pp. 83-92, but the operating mode and control characteristics of the single switch power converter and DCM rectifier are different.
Another controlled rectifier approach for alternators is described in U.S. Pat. No. 5,793,625, entitled “Boost Converter Regulated Alternator,” issued Aug. 11, 1998 to Thomas W. Balogh and assigned to Baker Hughes, Inc. The Balogh patent describes a circuit which utilizes boost mode regulator techniques to regulate the output of an ac source with this circuit. The source inductance becomes part of the boost mode circuit, thus avoiding the losses associated with the addition of external inductors. When a three-phase alternator is the power source, the circuit comprises a six diode, three-phase rectifier bridge, three field effect transistors (FETs) and a decoupling capacitor. The three FETs provide a short circuit impedance across the output of the power source to allow storage of energy within the source inductance. During this time, the decoupling capacitor supports the load. When the short circuit is removed, the energy stored in the inductances is delivered to the load. Because the circuit uses the integral magnetics of the ac source to provide the step-up function, a relatively efficient circuit is provided. The duty cycle of the switches (operated together) is used to regulate the alternator output voltage or current. The rectifier can thus be used to regulate the output voltage and improve the current waveforms for low-reactance machines that would otherwise operate with discontinuous phase currents.
While regulating output voltage or current with a boost circuit of this type may be useful in permanent magnet alternators having relatively low inductance characteristics, this method is not useful with alternators having a relatively large inductance characteristic and a wide operating speed range such as in wound-field Lundell-type alternators for automotive applications.
To understand this, consider that in a system which includes an alternator coupled to a boost rectifier, the output voltage is fully controllable by the boost rectifier when the internal machine voltages are the same magnitude or lower than the dc output voltage as described, for example, in the above referenced Venkataramanan paper. However, if the internal machine voltages become significantly larger than the desired dc output voltage, then the output voltage cannot be regulated by the boost rectifier independent of load without inducing unacceptably high currents in the machine. The typical automotive Lundell alternator presents this problem.
At the present, high-reactance Lundell-type alternators with diode rectifiers and field control are widely used in the automotive industry. Moreover, there is a very large infrastructure dedicated to the manufacture of Lundell-type alternators. However, design of these alternators is becoming increasingly more difficult due to continually rising power levels required in vehicles and in particular required in automobiles.
As is also known, the average electrical load in automobiles has been continuously increasing for many years. The increase in electrical load is due to the demand to provide automobiles and other vehicles with increasingly more electronics and power consuming devices such as microprocessors, electric windows and locks, electromechanical valves, and electrical outlets for cell phones, laptop computers and other devices. Such additional electronics results in a need for more electrical energy in automobiles and other vehicles.
Because of this increase in electrical load, higher power demands are being placed on automotive alternator systems. The great demand for increased output power capability from alternators has led to development of improvements over the simpler approaches. One widely-used method for improving the high-speed output power capability of alternators is the introduction of third-harmonic booster diodes.
A system which utilizes this technique is described in conjunction with FIG. 1B in which like elements of FIG. 1A are provided having like reference designations. As illustrated in FIG. 1B, in this technique, the neutral point of the Y-connected stator winding is coupled to the output via a fourth diode leg 18. While the fundamental components of the line-to-neutral back voltages are displaced by 120° in phase, any third harmonic components will be exactly in phase. As a result, third harmonic energy can be drawn from the alternator and transferred to the output by inducing and rectifying common-mode third harmonic currents through the three windings. The booster diodes in leg 18 provide a means for achieving this. In particular, at high speed the combination of the third harmonic voltages at the main rectifier bridge (at nodes a, b, and c in FIG. 1B) combined with the third harmonic of the back voltages are large enough to forward bias the booster diodes and deliver third harmonic energy to the output. In systems with significant (e.g., 10%) third harmonic voltage content, up to 10% additional output power can be delivered at high speed. Additional power is not achieved at low speed (e.g. at idle) using this method, since there is insufficient voltage to forward bias the booster diodes in leg 18.
The output power capability at idle speed is an important characteristic of an automotive alternator, and can be the dominant factor in sizing the alternator. Approaches which can improve the output power capability of an alternator at idle utilizing simple controls are thus of great value.